The present invention relates to a valve control arrangement. More particularly, it relates to a valve control arrangement for controlling closing and opening time of a valve in a displacement piston-internal combustion engine, the valve actuatable by a valve control cam of a cam shaft via an axially displaceable valve plunger.
Valve control arrangements of the above mentioned general type are known in the art. One of the valve control arrangements is disclosed in the German document DE-OS No. 3,125,650 and designed so that at the beginning of the stroke of the valve control cam which acts for valve opening, the opening of the stroke transmitting chamber is blocked. By the pressure of the valve control cam upon the stroke transmitting chamber, on which also the valve plunger abuts under the force of the valve closing spring in an opposite direction, no pressure medium can discharge from the stroke transmitting chamber. Thereby, the stroke movement of the valve control cam which is produced by the rotary movement of the cam shaft is completely transmitted to the valve plunger which in turn lifts the valve member from the valve seat and opens the valve.
The closing of the valve is performed at a predetermined point of time so that the opening of the stroke transmitting chamber is performed in a shock-like manner. Under the action of the valve control cam which presses on the stroke transmitting chamber, on the one hand, and the valve seat which presses on the stroke transmitting chamber, on the other hand, the pressure medium flows out of the pressure transmitting chamber and therefore its axial extension reduces. Despite further stroke movement of the valve control cam in direction of the valve opening, the valve plunger can move under the action of the valve closing spring in direction toward the valve control cam, and thereby close the valve. The stroke of the valve plunger or the valve member is shown in FIG. 2 vs the rotary angle of the valve control cam. The curve I represents the course with closed opening of the stroke transmitting chamber, and the curve II represents the course with release of the opening of the stroke transmitting chamber at point of time .phi..sub.SII. In dependence upon the fixation of the closing point of time .phi..sub.SII, the quantity of fuel aspirated into the cylinder can be adjusted in correspondence with different consumption in different operational conditions.
As can be seen from curve II in FIG. 2, after closing of the valve in point time .phi..sub.SII, a relatively long evacuation phase takes place, in which with the valve closed, the piston moves further downwardly in the cylinder and thereby a negative pressure is produced in the cylinder. Because of this negative pressure, the fuel cools relatively too much, the fuel evaporates poorly and as a result a poor mixture preparation takes place. The poor mixture preparation is a cause for a high hydrocarbon content in waste gas. In addition, the lower temperature fuel results in reduced temperature in exhaust, whereby the post-combustion of the waste gas in the exhaust is lower and the hydrocarbon content additionally increases. During idle running or in the lower partial load direction, the worsening of the mixture preparation because of the additional low flow turbulence with the fuel mixture aspiration and the shorter valve opening time is especially high.